1. Field of the Invention
The present invention relates to a wide area Ethernet network which utilizes a plurality of types of transmission networks to relay different Ethernet networks to one another, and more particularly, to an alarm transfer method for transferring fault occurrence information resulting from a line fault, a failed device and the like to a terminal device with which a communication is being made, and to a wide area Ethernet network to which the alarm transfer method is applied.
2. Description of the Related Arts
The proliferation and advancement of the Internet, intranets, and portable telephones cause a year-by-year increase in the traffic of speech and data which flow through networks. In such an environment, enterprises and service providers are imminently driven to build a wide area Ethernet network which supports the ever increasing traffic with a limited cost.
The wide area Ethernet network is built, for example, by use of an existing SONET/SDH network as a relay section to interconnect LANs, each implemented by an Ethernet network in such a manner that they can communicate with one another. For transmission in such a configuration, higher-level protocol data transferred on a LAN is encapsulated at an ingress node, which is the entrance of the wide area Ethernet network, in accordance with a protocol employed in a relay section, and is decapsulated at an egress node, which is the exit of the wide area Ethernet network. Techniques for such transmission in the wide area Ethernet network have already been brought into practical use, examples of which may be PPP over SONET (IETF RFC2615 standard), GFP (ITU-T G.7041 standard), and the like, as is known in the art. For example, when a SONET/SDH network is utilized as a relay section, encapsulated data is stored in a payload of a frame defined in the SONET/SDH network, and transferred by cross-connecting communication channels with a band which is previously set.
In the SONET/SDH network, the frame used therein comprises two types of overhead fields: SOH (Section Over Head) and POH (Path Over Head) (see, for example, ITU-T Recommendation G.707 (October 2000)). SOH is provided for managing a section which is defined as a transmission system portion of a transmission medium, while POH is provided for managing a path network layer which is independent of the transmission medium. In this way, the section and path are organized in a layered structure, and a plurality of paths are multiplexed in a payload, such that a transmission network can be organized in a layered structure comprised of a transmission medium network layer and a path network layer. This results in the ability to manage the designing, maintenance and operation of the network in a layered structure, thereby providing advanced network services. For example, since each relay node on a transmission path separately monitors SOH fault information and POH fault information, it is possible to readily find whether a communication fault, if any, is occurring in a section between relay nodes or only on a particular path, and to identify the location at which the fault is occurring.
However, since the SONET/SDH network has a large number of items to be monitored using SOH and POH, the maintenance and operation cost tends to increase. To limit the maintenance and operation cost, less expensive Ethernet networks represented by Fast Ethernet (hereinafter abbreviated as “FE”) and Gigabit Ethernet (hereinafter abbreviated as “GbE”) have been increasingly employed in wide area Ethernet networks for use as relay sections thereof. Alternatively, another wide area Ethernet network may include relay sections which may be implemented by a combination of an existing SONET/SDH network and an Ethernet network such as a GbE network, such that the SONET/SDH network is utilized in a certain section, while the GbE network is utilized in another section.
In the wide area Ethernet network, there are many clients which request a guaranteed transmission bandwidth and communication quality. To meet such a request, a protection function is needed for switching to a spare line if a line fault occurs. Also, alarm information indicative of the occurrence of a fault must be notified to clients placed in communication without fail. Further, since a large capacity of data is flowing through lines, a need exists for redundant transmission paths in relay sections and a reduction in a switching time upon occurrence of a fault.
In the aforementioned GFP, a client management frame shown in FIG. 1 is defined for transferring an alarm for notifying the occurrence of a fault such as a line fault, a failed device, and the like. FIG. 1 is a schematic diagram showing a format for the client management frame defined in the GFP.
As shown in FIG. 1, in the GFP, a communication partner can be notified of a fault occurring on a client line (for example, on an Ethernet line) using a UPI (User Payload Identifier) field in the client management frame. The UPI value indicates loss of signal (LOS) when it takes “00000001” and indicates link-down on a client line when it takes “10000000.” In a normal state where no fault is present, the UPI value is set to a value other than the foregoing “00000001” and “1000000.”
The client management frame is transferred to a communication partner at predetermined intervals while loss of signal or link-down is being detected. In different systems, the client management frame may be transferred to the communication partner as required, or transferred at predetermined intervals in a normal state where no fault is present.
A core header field shown in FIG. 1 includes such information as a source address, a destination address, and a priority, and a PTI (Payload Type Identifier) field shows how the frame is used. In FIG. 1, the PTI value is set to “100” which indicates that the frame is used as a client management frame. A PFI (Payload FCS Indicator) field shows whether or not FCS (Frame Check Sequence) is executed. The FCS is used for detecting transmission errors of payloads. An EXI (Extension Header Identifier) field stores an identifier of an extension header when it is used. In FIG. 1, the PFI value is set to “0” which indicates that the FCS is not executed, and the EXI value is set to “0000” which indicates that no extension header is used.
Next, a conventional alarm transfer method will be described with reference to a wide area Ethernet network, given as an example, which utilizes a SONET/SDH network as illustrated in FIG. 2. FIG. 2 is a block diagram illustrating an exemplary configuration of a conventional wide area Ethernet network.
The wide area Ethernet network illustrated in FIG. 2 comprises a plurality of Ethernet networks (two in FIG. 2) which are connected through a plurality of relay devices (two in FIG. 2) which make up the SONET/SDH network. The Ethernet networks are connected to relay devices 203, 204 of the SONET/SDH network through Ethernet termination devices 202, 205 contained in the respective Ethernet networks. Each of the Ethernet networks accommodates a plurality of client terminal devices (one each at the respective ends in FIG. 2. Hereinafter called the “client terminal”). Ethernet termination devices 202, 205 each time-division-multiplex data transmitted from the respective client terminals to generate SONET/SDH frames which are transmitted to relay devices 203, 204. Upon receipt of a SONET/SDH frame from relay device 203 or 204, the frame is demultiplexed into data for respective client terminals, and demultiplexed data are transmitted to the associated client terminals.
For transmitting data from an arbitrary client terminal to an opposing client terminal, a direction in which the data is transmitted is defined as a forward direction, and a direction opposite to the forward direction is defined as a backward direction. Also, a client terminal which originates data is said to be located upstream, while a client terminal which receives the data is called to be located downstream.
Each of devices which relays a communication between client terminals is collectively called the “relay node,” and particularly, a node which receives a signal from a client line or client circuit is called an “ingress node,” and a node which delivers a signal to a client line or client circuit is called an “egress node.” In the configuration of FIG. 2, assuming that the data source is client terminal 201, and the data destination is client terminal 206, Ethernet termination device 202 is in position of the ingress node, while Ethernet termination device 205 is in position of the egress node. A data flow from a client node serving as the data source to a client node serving as the data destination through respective relay nodes is called an “Ethernet path.” Relay nodes passed by a certain Ethernet path, and their port numbers have been previously set in the respective relay nodes. During a normal operation, an Ethernet path will not change to pass different relay nodes from previously set ones.
In the configuration as described above, assuming that a fault occurs, for example, on the client line between client terminal 201 in position of the data source and Ethernet termination device 202 to result in loss of signal or link-down, Ethernet termination device 202, which has detected the fault, transmits a client management frame indicative of loss of signal or link-down to Ethernet termination device 205 which is connected to client terminal 206 of the communication partner that is set in an Ethernet path. The client management frame indicative of loss of signal or link-down is transmitted at predetermined intervals while loss of signal or link-down is being detected.
Upon detection of the client management frame, Ethernet termination device 205 stops delivering a signal to associated client terminal 206, and forcefully sets the line to loss of signal or link-down. This state is maintained at all times while Ethernet termination device 205 is detecting the client management frame indicative of loss of signal or link-down.
In this way, as a fault occurs on a client line, line fault information is communicated to an opposing client line, so that the client lines appear to be directly connected to each other without causing client terminals 201, 206 to be aware of the existence of the intervening SONET/SDH network. This function for allowing the client terminal not to be aware of the existence of an intervening line or circuit is called “link-pass-through.”
For providing redundant relay lines in the SONET/SDH network, SONET/SDH has provided a mechanism for realizing a switching from a failed line to a spare line within 50 milliseconds upon occurrence of a fault. For operating this mechanism, an SOH field has one byte each of K1 byte and K2 byte which are communicated to an opposing communication device that serves as a communication partner (see, for example, ITU-T Recommendation G.841 (October 1998)).
In another example of providing redundancy for Ethernet lines wherein redundant lines are provided between two communication devices, an active transmission line is set to link-down in response to detection of a fault in an active system to notify an opposing communication device of the fault on the active transmission path. Upon detection of link-down, the opposing communication device switches the active system to a redundant system (see, for example, ITU-T Recommendation G.707 (October 2000)).
The conventional wide area Ethernet network which applies the aforementioned GFP has a problem of the inability to accomplish the link-pass-through for transferring information on a fault on a relay line to a client terminal of a communication partner when an Ethernet path is relayed through a plurality of types of transmission networks, as is the case with a combination of the GbE network and SONET/SDH network. This is because a field for use in transferring an alarm provided in the client management frame defined by GFP can merely transfer information on a fault which has occurred in a client line section.
As an example, when relay devices for building the GbE network are installed between the Ethernet termination devices and the relay devices of the SONET/SDH network illustrated in FIG. 2, and a line fault occurs, for example, between the upstream Ethernet termination device and a relay device in the GbE network, this fault information cannot be transferred downstream using the client management frame because the fault information notifies a fault on a relay line. If the UPI field in the client management frame were used to transfer information on loss of signal or link-down caused by a fault on a relay line in a manner similar to a fault on a client line, the downstream Ethernet termination device could not distinguish a client line fault from a relay line fault. This will cause a problem in identifying the location of the fault. The relay device in the GbE network is similar to the Ethernet termination device in that it time-division multiplexes data received from a plurality of Ethernet termination devices for transmission to the SONET/SDH network, and demultiplexes frames received from the SONET/SDH network into individual data for transmission to associated Ethernet termination devices.
To solve the above inconvenience, in a wide area Ethernet network which utilizes the GbE network and SONET/SDH network, inherent alarm transfer systems defined in the GbE network and SONET/SDH network may be used as they are to sequentially transfer the alarm information to downstream client lines.
For example, in a wide area Ethernet network comprised only of the SONET/SDH network illustrated in FIG. 2, as a fault occurs between relay devices within the SONET/SDH network, the fault information is transferred to a downstream Ethernet termination device through a path AIS (Alarm Indication Signal) alarm defined by SONET/SDH, and also transferred to an upstream Ethernet termination device through a path RDI (Remote Defect Indication) alarm defined by SONET/SDH. Ethernet termination device 205 implements the link-pass-through by setting an Ethernet line to link-down only for those client terminals which utilize a path for which the path AIS is detected. Similarly, Ethernet termination device 202 implements the link-pass-through by setting an Ethernet line to link-down only for those client terminals which utilize a path for which the path RDI is detected.
However, assuming that the alarm transfer systems inherent to the GbE network and SONET/SDH network are used as they are for sequentially transferring alarm information to downstream client lines in a wide area Ethernet network which has relay devices for building the GbE network, each installed between each of the Ethernet termination devices and each of relay devices of the SONET/SDH network as illustrated in FIG. 2, a fault in the GbE section would cause forced disconnection of not only a line which connects a failed Ethernet termination device to a relay device of the GbE network but also lines, each of which connects a normal relay device of the GbE network multiplexed with the relay device involved in the fault to a relay device of the SONET/SDH network. As a result, a normal Ethernet path from another Ethernet termination device which passes through the relay device of the GbE network is also forcefully set to link-down.
On the other hand, in a wide area Ethernet network which stores a GFP capsule in a data field of a MAC frame such as GbE as it is for transfer, there is a problem of the inability to organize a transmission network in a layered structure comprised of a transmission medium network layer and a path network layer for designing, maintaining, and operating the network. This is because no field is defined in the MAC frame used in the Ethernet network for distinguishing the transmission medium network layer from the path network layer, thus failing to manage the section and path in a layered structure, as is done in the SONET/SDH network.
The MAC frame is comprised of a preamble field (seven octets long); an SFD (Start Frame Delimiter) field (one octet long) for frame identification; a destination MAC address (hereinafter abbreviated as “DA”) field (six octets long); a source MAC address (hereinafter abbreviated as “SA”) field (six octets long); a LENGTH/TYPE field (two octets long) indicative of the frame length or type; a data field (46 to 1,500 octets long); and an FCS (Frame Check Sequence) field (four bytes) for CRC operation. Errors can be detected in received frames by monitoring the FCS field.
However, the MAC frame lacks for a field for distinguishing the section from the path for management, so that even if an error is detected in the CRC operation with the FCS field, it is not possible to immediately identify whether the error is associated with a fault in a transmission medium network layer or with a fault in an individual path network layer independent of a transmission medium. For the identification, the detected error must be compared with fault information generated by the preceding and subsequent relay nodes.
Further, in a wide area Ethernet network which includes relay sections comprised of a combination of an existing SONET/SDH network and an Ethernet network such as a GbE network, such that the SONET/SDH network is utilized in certain sections and the GbE network is utilized in the remaining sections, when an Ethernet network switching means used for providing redundancy for both the SONET/SDH network and Ethernet network involves forcefully bringing an active system, which detects an existing fault on a transmission path, to link-down to notify the fault on the transmission path to an opposing communication device for switching the system, a relay node which serves as a junction of the SONET/SDH network with the Ethernet network must be provided with two different types of switching means, i.e., a switching means which uses the K1 byte and K2 byte of the SONET/SDH network, and a switching means which uses link-down of the Ethernet network, thus resulting in an increase in the circuit scale, mounting area, and switching time.